Purification of benzene

ABSTRACT

HIGH PURITY BENZENE CAN BE PRODUCED FROM BY-PRODUCT AROMATIC STREAMS RICH IN BENZENE BY THERMAL TREATMENT OF THE CRUDE BENZENE-RICH BY-PRODUCT AROMATIC MATERIAL IN ADMIXTURE WITH ALKYL AROMATIC SUCH AS TOLUENE, UNDER HYDRODEALKYLATION CONDITIONS IN THE ABSENCE OF ANY CATALYTIC MATERIAL. THE PROCESS PERMITS THE PURIFICATION OF THE CRUDE BENZENE SIMULTANEOUSLY WITH THE HYDROALKYLATION OF THE ALKYL AROMATIC COMPONENT TO BENZENE.

a. R. JUNGERMAN ETAL 3,564,069

PURIFICATION OF BENZENE Filed Oct. 22, 1969 INVENTORS. seams RJUNGERMFM/ DOA/0L0 n. Rum/2o BOBBY A. WEfll/ER Eowuv H. IVEZJR.

United States Patent U.S. Cl. 260-672 8 Claims ABSTRACT OF THE DISCLOSURE High purity benzene can be produced from by-product aromatic streams rich in benzene by thermal treatment of the crude benzene-rich by-product aromatic material in admixture with alkyl aromatic such as toluene, under hydrodeal-kylation conditions in the absence of any catalytic material. The process permits the purification of the crude benzene simultaneously with the hydrodealkylation of the alkyl aromatic component to benzene.

This is a continuation-in-part of our application Ser. No. 648,707 filed by us on June 26, 1967, now abandoned.

This invention relates to a method of producing benzene of a high degree of purity from by-product aromatic streams, as hereafter defined, by feeding such streams, together with additional toluene, xylene, trimethyl benzenes or a mixture thereof and hydrogen in a ratio of about 2 to 20 moles of hydrogen per mole of hydrocarbon, preferably about 4-6 moles of H per mole of hydrocarbon, into a thermal hydrodealkylation zone at a temperature of from about 1100 to about 1500 F, preferably 1225 to 1350 F. at a pressure of from about 250 to 900 p.s.i.g., preferably 450 to 600 p.s.i.g., with a reaction time of from about 1 to about 120 seconds, preferably 20 to 50 seconds, and then cooling the reaction mixture by use of a high boiling quench liquid or indirect heat exchange with the feed stock.

By-product aromatics as used herein are complex liquid mixtures of hydrocarbons obtained from cracking of petroleum or naphtha fractions or from the dehydrogenation of light hydrocarbons, such as propane or ethane. The by-product aromatics may contain some benzene, toluene, xylenes, trimethyl benzenes and heavier aromatics such as polycyclic aromatics, including biphenyl and naphthalene and alkylated derivatives thereof. In addition to the aromatic components, the mixture can contain up to about 20% by volume of aliphatic or cyclic nonaromatic hydrocarbon impurities. The impurities are not readily separable from benzene by distillation. Included among the unsaturated hydrocarbon contaminants in the crude aromatic hydrocarbon stream are 1 to about 6 weight percent of the stream of dienes which tend to polymerize when heated to form tars or co-kes.

Prior to this invention recovery of pure benzene from the by-product aromatics could be effected only by multistep, expensive processes such as by acid wash treatment and subsequent careful fractional distillation of the benzene, or by selective extraction with ethylene glycol, a glycol ether, dirnethylsulfoxide, sulfolanc, or other selective solvent for aromatic compounds, separation of the hydrocarbons and thereafter carefully fractionating the hydrocarbons. In either of the above procedures the aromatic compounds other than benzene are either burned as fuel or added to gasoline. The aliphatic components in some cases are recycled through the cracking stage or are burned as waste. Attempts to hydrodealkylate the ice by-product aromatics have resulted in serious coking problems due to the formation of small amounts of carbon, which then appear to accelerate additional coking or carbonization, so that frequent shutdowns are needed to clean the carbon out of heat exchangers and/or the reactor. One method for purifying BTX streams under hydrode alkylating conditions with H in the presence of a catalyst is described.

It has now been found that if sufficient toluene, xylene or trimethyl benzene or a mixture thereof is added to the crude by-product aromatics to provide a feed containing 3080% toluene, xylene, trimethyl benzene, or a mixture thereof, mixing this feed with 2-20 moles, preferably 4 to 6 moles of hydrogen per mole of hydrocarbon, heating the mixture to 1100-1500" F., preferably 1225 F. to 1350 F. under a pressure of 250 to 900 p.s.i.g., preferably 450- 600 p.s.i.g., and holding the mixture at the above temperature for 1120 seconds, preferably 20 to 50 seconds, without a catalyst, the non-aromatic compounds are converted to compounds which are readily separated from the eflluent and the alkyl monocyclic aromatic hydrocarbons are hydrodealkylated, to a large extent, to produce more benzene. The reactor eflluent is passed through a clay tower to polymerize small amounts of unsaturates and pure benzene is separated by fractional distillation. Bottoms from the benzene distillation are sent to a toluene or feed distillation unit to which is added the requisite amount of by-product aromatic and make-up alkyl aromatic and the mixture heated to form a vapor having the composition desired as feed for hydrodealkylation or a portion of the bottoms can be blended with the feed to the dealkylating unit. Alternatively, the toluene recovered can be used for adjusting the toluene content of the crude by-product aromatic feed by blending both liquids for cycling through the process. After the hydrodealkylation step some light aliphatic hydrocarbons are flashed off with excess hydrogen. The flashed gaseous ingredients can be treated to remove substantially all the light hydrocarbons except methane and a part of the mixture of H and CH can be recycled to the reactor. Usually this gaseous recycle stream contains from about 50 to 65% by volume of H and the remainder essentially methane.

The by-product aromatic stream composition can be adjusted by distillation and/or the addition of toluene or xylene or a mixture thereof, preferably toluene, to provide a feed containing 30 to by volume of toluene, xylene, trimethyl benzene or mixtures thereof. When the feed composition is within the limits described and the reacton conditions and hydrogen-hydrocarbon ratios are within the defined ranges, the reaction can be run for very long periods of time without carbon build-up in the reactor or efiluent parts.

An object of the invention is to provide a method for preparing high purity benzene.

Another object is a method for preparing benzene of purity above 99% by reacting a crude by-product aromatic composition in admixture with an alkyl monocyclic aromatic compound under hydrodealkylating conditions.

Another object is a method of preparing high purity benzene without substantial carbon formation by reacting a crude by-product aromatic hydrocarbon stream in admixture with 30-80% by volume of an alkyl aromatic compound, such as toluene, xylene, or trimethyl benzene or mixtures thereof and a molar excess of hydrogen under hydrodealkylating conditions.

Further objects and advantages of the invention will be apparent from the detailed description which follows.

EXAMPLE 1 The detailed description of the invention which follows is intended to illustrate but not to limit the invention. Reference is made to the flow diagram to more clearly illus trate the invention. The crude by-product aromatic stream used in this description was obtained from a process for producing ethylene by cracking a mixture of ethane and propane. The said stream contained about 90% by volume of benzene, about 5 mole percent of aromatic compounds heavier than benzene, such as toluene, naphthalene, xylene, and alkylated naphthalene, and approximately 5 mole percent of C to C aliphatic hydrocarbons and cycloaliphatic compounds, such as dicyclopentadiene, cyclohexane, cyclohexene, methylcyclohexane and cyclopentadiene and methylcyclopentadiene. The ingredients in the feed, efiluent, intermediate stages, and purified products in the procedure are given in average moles per day of operation.

A mixture of crude by-product aromatic feed containing about 3400 moles benzene, 170 moles of the light fraction described above containing typically up to 5% dienes, including 1.5 to 3% methylcyclopentadiene, .1 to 1.5% is cyclopentadiene and .l to 2% of a mixture of pentaand hexadiene, based on the mixture, and 11,056 mols of toluene was fed through line 10, through heat exchanger 11, which contained hot efiiuent from reactor 12, as hereafter described. The heated crude by-product aromatic-toluene mixture was fed through line 13 to line 14 which led to about the center of a toluene still 15. Thus, this process is a description of a procedure wherein the feed to the reactor is controlled by the distillation procedure. It is to be understood, however, that the toluene still can be used to supply relatively pure toluene which can be blended with the crude liquid by-product aromatic in line 20 before it enters reactor 12. The temperature of the toluene still is maintained so that the vapors will contain 30 to 80 mole percent of toluene together with all the benzene, C -C aliphatic or non-aromatic cyclic fraction and a small part of the heavier aromatics from the benzene column bottoms. The toluene still 15 is equipped with a reboiler 16 to maintain the temperature at the desired level. Bottoms from the still are removed through line 17. The vapors from still 15 are condensed and a portion of the liquid is then fed through line 18 at a pressure of about 550-600 p.s.i.g. (pump not shown) to line 19. The remaining portion of the liquid is fed to still 15 through line 62 as reflux, the low boiling hydrocarbons are bled off through line 63. Hydrogen containing about 30 mole percent methane is fed through line 33 at a pressure of 550600 p.s.i.g. into line 19. Thus, the alkylated monocyclic aromatic hydrocarbon, crude by-product aromatic and hydrogen are blended in line 19 before being fed through heat exchanger 22, through which hot efiluent from reactor 12 passes to preheat the feed mixture. The latter enters line 23 and passes into a direct fired pre-heater 24, where the temperature is raised to about 1200-1280 F. The mixture leaves pre-heater 24 through line 25 and is fed into the reactor 12. At this pre-heat temperature range an exothermic reaction between the hydrogen and the feed takes place to raise the temperature to 1225-1350 F. in the reactor 12. The effluent from the reactor passes through line 26 into heat exchanger 22, and line 28 into heat exchanger 11 and then to line 29 and cooling units, not shown and then into a pressurized decanter 30. A portion of the cooled liquid from decanter 30 is fed back to the base of reactor 12 through line 31 to cool the hot gases to a temperature of 1200 F. or below to stop or at least slow down the reaction. Light hydrocarbon, rich in methane, and hydrogen are vented under controlled pressure conditions through line 32 in the top of decanter 30. Line 65 contains a pressure control valve V through which some of the mixture can be sent to the lean oil scrubber through line 41. The mixture of hydrogen and light hydrocarbons are compressed to provide a mixture of 45-65 mole percent hydrogen and the remainder substantially methane. This mixture of hydrogen and methane is recycled at the proper pressure to the reaction through line 33. Any make-up hydrogen needed is fed to the reaction system through line 34, also at the appropriate pressure. The liquid from high pressure decanter 30 is passed through line 35 with proper pressure controls to low pressure decanter 36. Liquid now rich in benzene is passed from low pressure decanter 36 through line 37 to storage tank 38. Low pressure vapors from decanter 36 are passed through line 39 to compressor 40 and through line 41 to the lower portion of a lean oil scrubber 42 where the aromatic compounds are absorbed in an oil such as kerosene and the normally gaseous ingredients are vented through line 43 to a fuel system to provide heat for pre-heat furnaces of the system. The bottoms from scrubber 42 are fed to a distillation column, through line 44. From this column the vapors rich in benzene are passed through line 45 to heat exchanger 46 and then to storage tank 38. Appropriate pumping systems are not shown. The bottom of this column has a reboiler 47. Bottoms from the column pass through line 48 and are fed back to scrubber 42.

The benzene rich liquid from storage tank 38 is fed through line 49 to pump 50, which propels the liquid through line 51 to a pre-heat furnace 52, where the mixture is heated to 350 to 450 F. This heated mixture leaves the furnace through line 53 and is fed to a clay packed tower 54 where small traces of non-aromatic unsaturated hydrocarbons are polymerized. From the clay tower the liquid passes through line 55, into a benzene distillation column 56. Here the benzene is fractionated and removed through line 57 into separator-condenser 58. Small amounts of gaseous products are vented as fuel through line 59 and a portion of the liquid in condenserseparator is returned to column 56 as reflux and the remainder is withdrawn through line 60 as high purity benzene of nitration .grade analyzing 99.5% or more benzene. The bottoms containing primarily some toluene and higher aromatics are fed to toluene still 15, through lines and 14. Any unreacted toluene, xylene or trimethyl benzenes are thus recycled with additional crude by-product aromatics to repeat the series of steps described.

Typical analyses of the crude by-product aromatic streams obtained from cracking ethane propane mixtures are:

Percent Hexene .0-.5

Cyclopentene 0-1.5 Cyclohexene .5-3 Hexadiene O-1 Heptene 0-.5 Cyclohexane .1-1 Cyclopentadiene .11.0 Methylcyclopentadiene 14 Dicy-clopentadiene 01.5 Toluene 0-10 Ethylbenzene 0.1-0.5 Styrene 01 Benzene Balance 98.3-74.5

The components most difficult to separate from benzene are cyclohexene and methylcyclopentadiene, and their presence is the most troublesome because they are deleterious in the preparation of ethyl benzene for use in making styrene. Previous attempts to remove the cyclohexene and 'methylcyclopentadiene by thermal treatment of by-product aromatics resulted in serious polymerization and coking problems.

Tabulated below are data from a 24-hour operation of the process. The crude benzene stream was obtained from dehydrogenation of a mixture of ethane and propane. It contained by volume 5% toluene, .09% cyclohexane, 1.5% cyclohexane and 2.5% methylcyclopentadiene. The reaction temperature was l3251350 F. and the pressure was 500600 p.s.i.g.

TABLE 1 Combined Crude Combined Gas Fresh total Make-up Recycle Recycle benzene fresh to Benzene toluene 7 feed Hz gas liquid feed feed Efiluent fuel product feed H2 mo1s 71, 000 17, 400 57, 950 4, 350 CH4 mols. 42, 140 8 54, 320 13, 050 Benzene mo1s 3, 400 14, 100 14, 100 Toluene mols 12, 970 1, 990 70 10, 886 Heavies mols 60 510 Lights mols 170 100 100 Total mols 129, 740 18, 270 94, 870 1, 974 3, 740 14, 626 138, 970 17, 600 1 14, 170 10, 886

1 99.5% or higher purity.

NOTE.Residence time=24.6 sec.; Yield, vol. pereent=81.2 benzene from toluene in fresh feed; Conversion pereent=84.7 (conversion of toluene).

EXAMPLE 2 In this 24 hour run 34,335 gallons of a crude benzene stream containing by volume, 3.3% CSs, 3.0% C6s, 1.2% C7s, including 0.1% toluene, and about 92.5% benzene and 117,934 gallons of toluene plus 11,671 gallons of benzene were fed to the hydrodealkylation reactor at an inlet temperature of about 1230 F. and a pressure of 520 p.s.i.g. The mixed feed contained about 26.5 volume percent of benzene. During the reaction, 58,400 moles of recycle hydrogen, containing in addition about 43,700 moles of CH, and C H and 16,340 moles make up hydrogen, containing about 660 additional moles CH and C H were added to the feed before it entered the pre-heat furnace. The reac-tion mixture was quenched with a portion of the liquid condensate from the reaction mixture. The 162,000 gallon effluent on cooling contained about 83.2% by volume of benzene. The recycle stream from the benzene column consistedof 13,665 gallons of toluene and 1373 gallons of benzene. The bottoms from the toluene column amounted to 9,727 gallons per day. A portion of these toluene column bottoms which contain some diphenyl, alkylated biphenyls and naphthalenes can be recycled with the toluene-crude benzene feed by addition to a stream in line 18. The residence time was calculated as 23.2 seconds. Based on these data it is calculated that the conversion of toluene was 89.2 mole percent and the yield or specificity was 91.9% of theoretical. The recovered benzene had a purity of 99.7%. The chief impurity was toluene.

The reaction has been operated for more than one month with 41.5% of crude benzene in total feed. The 4 crude benzene analyzed, by volume, 80.1% benzene, 13.9% toluene and 6% impurities which were mostly C5, C6 and C7 dienes, without excessive coke formation. Runs in excess of two months were made with C -C impurities in the crude benzene. Runs as long as 176 days between shutdowns to remove coke have been made at diene levels averagin about 3.1% by weight in the crude benzene. This included 1.7% methcyclopentadiene, 0.5 cyclopentadiene and the remainder was a mixture of penta and hexadienes.

If reactors are made with metal linings, H S or compounds convertible to H 8, in whole or in part, under the reaction conditions used can be added to inhibit metal flaking.

We claim:

1. A method of preparing benzene of high purity comprising blending a crude benzene-rich by-product aromatic mixture containing from about 1 to about 4 weight percent methyl cyclopentadiene and a total diene content of from 1 to 6 weight percent of said mixture obtained from cracking of petroleum or from dehydrogenation of ethane or propane with from about 2 to about 20 mols of hydrogen per mol of hydrocarbon, and an alkylated benzene selected from the class consisting of toluene, xylene, trimethyl benzene or mixtures thereof, to provide a feed stream containing 30-80% by volume of the said alkylated benzenes, heatin the blended ingredients to a temperature in the range of 1100 to 1500 F. at a pressure in the range of 250-900 p.s.i.g. for a period of from 1 to 120 seconds, in the absence of any catalyst and separating purified benzene from the so treated mixture.

2. The method of claim 1 in which the temperature ranges from 1225 to 1350 F., the pressure ranges from about 400 to about 600 p.s.i.g. and the reaction time ranges from about 20 to about seconds.

3. The method of claim 1 in which the byproduct aromatic is blended with hydrogen and toluene prior to entering the hydrodealkylation reactor.

4. The method of claim 1 in which a portion of the cooled liquid efiluent from the reaction is recycled to the bottom of the reactor as a quench stream.

5. The method of claim 1 in which the unsaturated compounds from the liquid reaction eflluent are polymerized prior to separation of the benzene from higher boiling compounds.

6. The method of claim 1 in which the normally liquid mixture fed to the reactor is adjusted as to its composition by controlling the composition of the condensate from the toluene distillation column to provide a mixture of byproduct aromatic and an alkyl aromatic compound selected from the class consisting of benzene, toluene, xylene, trimethyl benzene and mixtures thereof.

7. The method of claim 6 in which the said alkyl aromatic is toluene.

8. The method of claim 1 in which the normally liquid hydrocarbon feed to the reactor is prepared by blending liquid by-product aromatic with toluene.

References Cited UNITED STATES PATENTS tion, Hydrocarbon Processin 46(2) -158 (February 1967).

DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, Assistant Examiner US. Cl. X.R. 260674 

